Computer systems and other electronic equipment are commonly partitioned into subsystems on discrete Printed Wiring Boards (PWBs). These building blocks are typically multi-layer boards with discrete layers dedicated to signal, power and ground. Consequently, vias or plated through-holes (PTHs) are commonly used to connect signal traces from one layer of a PWB to another for connection to devices or other PWBs.
A connector is used to couple PWBs with one type of connector being a through-hole connector that utilizes pins which plug into the vias to connect to signal traces and ground traces on the PWB. The vias on the PWB are arranged in a pattern, called a connector footprint, the dimensions of the vias and the spacing between the vias are arranged to correspond to the arrangement of pins on the connector so that the pins of the connector can be plugged into the vias of the connector footprint. The connector footprint includes signal and ground vias which receive, respectively, signal and ground pins of the connector.
Cross-talk is a phenomena where a signal carried in one conductor causes a disturbance to the signal carried in another conductor. The conductor which causes the disturbance is called the aggressor and the conductor which experiences the disturbance is called the victim. The disturbance is caused by electromagnetic coupling between the two conductors and becomes more pronounced at high frequencies. Various techniques for eliminating cross-talk exist including shielding to prevent the disturbance from occurring and active compensation to cancel cross-talk that has occurred in the victim.
Cross-talk in state of the art back-plane connectors is a primary limitation in the operation of serial links at speeds in excess of 5.0 Gbps. Part of this cross-talk comes from within the connector, and part of it comes from the interface between the connector and the PWB.
High speed back-plane connectors usually do a good job of shielding signals from each other within the connector. However, often times, the design of the interface between the connector and the PWB does not prevent cross-talk for certain signals. For example, the VHDM(copyright) Connector, manufactured by Teradyne. The 8-row version of this connector has an interface depicted in FIG. 1. FIG. 1 shows signal traces labeled by a row designation (A-H) and a column designation (1xe2x88x92N) within the Teradyne VHDM(copyright) 8-row connector. Vertical ground strips separate adjacent signal columns. The ovals represent typical grouping of signal traces to form differential pairs. The ground strips provide shielding between the signal traces.
FIG. 2 shows the transformation of connector field to the footprint on the PWB. The signal traces end in signal vias in rows A-H, while the ground strips end up as columns of ground vias. In FIG. 2 the ground vias are labeled as rows J-P. Thus, any via in the footprint can be identified by a row and column designation. Signal pins on the connector plug in to the vias on the footprint to transfer signals on the PWB to other boards or cables.
Through hole connectors like the Teradyne VHDM(copyright) exhibit a significant amount of cross-talk from the connector footprint in the outer rows, e.g., rows A and H of the 8 row VHDM(copyright) connector due to insufficient shielding.
According to one aspect of the invention, an additional ground via is added to each end of the ground via columns in the connector interface on the PWB. The additional vias are not connected to ground pins of the connector.
According to another aspect of the invention, additional, unconnected ground vias are added to a connector interface footprint to provide additional shielding for preventing cross-talk between signal vias near the periphery of the connector footprint.
Additional features and advantages of the invention will be apparent in view of the following detailed description and appended claims.